The present invention relates to a light-emitting composition and a fluorescent lamp. 2. Description of the Related Art
In recent years, a demand has arisen for improvements in quantity and quality of light of fluorescent lamps. For this purpose, rare-earth phosphors having a high luminance and excellent color rendering properties have been used for fluorescent lamps.
Rare-earth phosphors are very expensive although they have high luminous efficacy with a high luminance. The cost of the rare-earth phosphor is 10 to 30 times that of a calcium halophosphate phosphor. Attempts have been made to reduce an amount of rare-earth phosphor used in a light-emitting composition. For example, the grain size of a rare-earth phosphor is reduced so that an amount of application can be reduced. However, when the grain size of the phosphor is reduced, a luminance is decreased. In addition, with small grain size, it is difficult to uniformly coat the inner surface of a bulb with a small amount of phosphor, and a bulb surface may be exposed due to a coating error. Further, since the thickness of a phosphor film is decreased, a large amount of sodium is precipitated from glass constituting a bulb during heating in the fabrication process of fluorescent lamps. As a result, a luminous flux maintenance rate of the fluorescent lamp is decreased and color shifting is increased. In addition, the bulb wall may be blackened and a phosphor layer may peel from the bulb wall.
A technique for decreasing an amount of phosphor used in the fabrication of the fluorescent lamp without degrading various characteristics of the lamp is described in Japanese Patent Disclosure (Kokai) No. 57-128452. This prior art describes a light-emitting composition obtained by mixing a predetermined inorganic white powder in a phosphor. However, even this light-emitting composition cannot provide satisfactory results in view of lamp characteristics over time when the composition is used in the fluorescent lamp. In particular, these characteristics are represented by the luminous flux maintenance rate, color shifting, and blackening of the bulb wall.